This invention relates to braking systems. In particular, this invention relates to multi-disk brake systems used in vehicles. More particularly, this invention relates to aircraft disk brake systems. The brake disk assembly contains a brake disk stack, which comprises an end plate, a pressure plate and interleaved rotors and stators. The available wear portions of the rotors and stators have different wear thicknesses, so that the thickest disks in the stack are capable of being used for three service runs before being refurbished or replaced. Similarly, the available wear portions of the pressure plate and end plate each may have a different available wear portion thickness, which may be the same as the available wear portions of the stators and rotors. Therefore, with this configuration, the mid-thickness disks are replaced or refurbished after two service runs and the thin disks are replaced or refurbished after one service run. Furthermore, this invention relates to a method of assembly of a disk brake stack having disks of different thicknesses.
Aircraft brake systems have brake disk stacks comprised of interleaved rotor and stator disks. The disks are alternatively splined to the rotatable wheel and stationary (non-rotatable) torque tube. An end plate is provided at one end of the stack, while an axially moveable pressure plate is provided at the other end of the stack. Upon application of force to the pressure plate, the disks frictionally engage and provide braking activity to the vehicle.
Generally in the past, brakes have been assembled with the available wear portions of the rotors and stators having the same available wear thickness. These brakes were operated until the wear portions of all of the disks were fully worn. At that time, all of the disks were removed and replaced with new unworn disks or refurbished or reworked disks. The heat sink mass of the brake is reduced by the total wear of all the disks in the brake stack. The heat sink mass must be above a certain level to hold the operating temperature of the brake down. Due to the size and weight limitations on aircraft, the thickness of available wear portions is also limited. The thickness of the available wear portions controls the number of landings between the replacement or refurbishing of the disks. The piston cylinder or other actuator assemblies for actuating the brakes also have a predetermined travel length, which is a function of the total wear of the disks. Where the brake is operated until all disks are fully worn, the actuator travel length is increased and accordingly the total brake envelope is increased as well as weight of the brake assembly. The brake envelope includes a heat sink envelope which is the distance between the end of the piston or thrust member facing the brake stack and the backing plate or reaction member at the opposite end of the brake stack.
Various different brake configurations have been proposed to minimize the turnaround time, maximize the brake cooling, reduce the amount of piston or actuator travel, as well as the size and the weight of the brake, while at the same time retaining a substantial portion of the heat sink mass to a lower operating temperature. For example, U.S. Pat. No. 3,480,115 to Lallemant discloses a brake, which comprises two groups of coaxial disks. The first group of coaxial disks is smooth since the disks are not equipped with a friction lining. The second group of disks has a friction lining. The disks of the first group are disposed alternatively with the disks of the second group. The disks of one group are angularly coupled to a rotatable structure such as an aircraft wheel and the disks of the other group are coupled to a non-rotating structure. Lallemant discloses various embodiments in which the disks of at least one of the two groups have thicknesses that vary from one disk to another depending upon the axial position of the disks. The thicker disks have greater thermal capacities due to their greater mass. According to Lallemant, the purpose of these various embodiments is to provide multiple disk brakes that fulfill the requirements of practice, particularly with respect to their longevity, uniformity of braking efficiency and conditions in which maintenance operations can be carried out. Furthermore, Lallemant contemplates the possibility of re-machining the smooth disks and moving these disks to a new axial position in the brake for which new axial position the reduced thickness that has been given to the smooth disks is adapted to the working conditions corresponding to the new position. Lallement alleges that the overall thermal capacity of the brake is not affected by the re-machining of the smooth disks and therefore, the efficiency of such brakes is constant.
U.S. Pat. No. 4,613,017 to Bok discloses a method of assembling and overhauling a disk brake having a plurality of disks with available wear portions of predetermined different thicknesses. The method comprises positioning first a first group of disks in overlapping relationship with a second group of disks. The first group of disks have an available wear portion of a first thickness and the second group of disks have an available wear portion of a second thickness which is greater than the thickness of the first group of disks. A third group of disks having a third thickness at an intermediate overhaul replace the first group of disks when the available wear portions of the first group of disks are substantially fully worn. The third group of disks have a third thickness which is greater than the thickness of each of the available wear portions of the second group of the disks at intermediate brake overhaul time.
Similarly, U.S. Pat. No. 4,742,895 to Bok discloses a carbon disk brake assembly. The assembly comprises a plurality of disks in which the first group of disks, for example, the stators and end plates, have an available wear portion of a first thickness which is less than and preferably one half of the thickness of the wear portions of the second group of disks, in this example, rotors. After a predetermined number of landings, the stators and end plates will be substantially fully worn. These worn stators and end plates are replaced by a third group of disks, which are new or refurbished stators and end plates. Preferably, the wear thickness of this third group of disks is double the thickness of the available wear portions of the rotors at the intermediate overhaul. The brake assembly is then operated to another intermediate overhaul when the available wear portions of the rotors will be fully worn and replaced by new or refurbished rotors.
U.S. Pat. No. 4,977,985 to Wells, et al. discloses a method of carrying out the maintenance of a multi-disk brake. The disks are made of a carbon-carbon material. The brake comprises a stack of interleaved rotor and stator disks provided between a thrust member and a reaction member. The rotor and stator disks are selected and arranged so that the wear allowance of a set of disks at one end of the stack is less than the wear allowance of a set of disks at the other end of the stack. A fully worn group of disks is removed from one end of the stack during brake service operation following wear. The remaining partially worn disks are axially moved or shifted to a new position towards the one end of the stack. An unworn stack of disks is added at the other end of the stack.
Similarly, U.S. Pat. No. 5,323,880 to Wells, et al. discloses a multi-disk brake system. The brake comprises a stack of interleaved rotor and stator disks disposed in an axially aligned relationship. The disks are made of a carbon-carbon material, which provides the frictional surfaces of the disk as well as their structural integrity. The stack of disks comprises a first group of adjacent rotors and stators and a second group of adjacent rotors and stators in an axially aligned relationship with the first group. Each wear surface of the first group which confronts a wear surface of an adjacent disk of the first group is unworn. Each wear surface of a disk of the second group which confronts a wear surface of an adjacent surface of the second group is partly worn. Each group has an end disk, which confronts an end disk of the other group. The confronting wear surfaces of the end disks are either both unworn or are both partly worn.
U.S. Pat. No. 5,509,507 to Wells et al. discloses a multi-disk brake system for aircraft. This brake system comprises a stack of interleaved rotor and stator disks made of carbon-carbon material and disposed in an axially aligned relationship between a thrust device and a reaction member. The stack comprises a first group of adjacent rotor and stator disks and a second group of adjacent rotor and stator disks with the two groups in an axially aligned relationship. Only one disk of the first group contacts a disk of the second group. Each group has an end disk which confronts an end disk of the other group. The wear surfaces of the disks of the first group are thicker than the wear surfaces of each of the disks of the second group. At an intermediate overhaul time, after a predetermined number of brake applications, each wear surface of the first group which confronts a wear surface of an adjacent disk of the first group is only partly worn away, whereas each wear surface of a disk of the second group which confronts a wear surface of an adjacent disk of the second group is substantially fully worn away. Also at the intermediate overhaul, the confronting wear surfaces of the end disks are either both only partly worn away or both substantially worn away.
U.S. Pat. No. 5,295,560 to Moseley discloses a thermally balanced brake stack. The brake stack comprises a plurality of stator disks, a plurality of rotor disks interleaved with said stator disks, a pressure plate at one end of the stack and an end plate at the other end of the stack. The rotor and stator disks at the first and second end are thinner than the rotor disks at the center portion of the stack. According to the patent, this configuration minimizes the rate of temperature increase at the center of the stack while enabling quicker dissipation of heat at the end of the brake disk stack.
A pressure balanced brake stack is disclosed in U.S. Pat. No. 5,551,534 to Smithberger et al. The brake stack comprises a plurality of rotor disks, a plurality of stator disks interleaved with said rotor disks, a pressure plate at a first end of the stack and an end plate at the second end of the stack. The rotor disks at the first and second ends of the stack are thicker than the rotor disks adjacent thereto. Due to this configuration according to the patentee, the rotor disks at the first and second ends will deflect less and distribute pressure more uniformly throughout the stack.
The present invention comprises a novel disk brake assembly construction. The disk brake is assembled with disks having three different wear portion thicknesses which enables the thickest disks to go through three service runs prior to being replaced or refurbished. A run is defined as a service operation, e.g., numerous brakings of a vehicle, between brake overhauls. A brake overhaul is made after a predetermined number of aircraft landings, or when the disks having the thinnest wear portions are fully worn. A brake wear limit indicator is preferably used to determine when a brake overhaul is necessary. Such wear limit indicators are known in the prior art. A common type uses a pin that retracts as the brake wears.
The brake stack includes, in addition to an end plate and pressure plate, rotors and stators. The end plate and pressure plate each have only one wear surface, while rotors and stators each have two wear surfaces. The rotors are interleaved with the stators. The rotors, stators, end plate and pressure plate are of three different sizes: a thick disk, a mid-thickness disk and a thin disk depending upon the configuration of the brake stack. The thick disks have an available wear portion of a first thickness. The mid-thickness disks have an available wear portion which is about two thirds of the available wear portion of the thick disks. The thin disks have an available wear portion which is about one third of the available wear portion of the thick disks.
Either all the rotors or all the stators in the brake stack are changed at each overhaul. Initially, in one embodiment, the brake stack can comprise thick stators and thin rotors and thick disks in the end plate and pressure plate positions. After the first service run, at the first overhaul, the thin rotors are removed and mid-thickness disks inserted into the rotor positions. The stators and the end plate and pressure plate which have been worn to become mid-thickness disks are reassembled into the overhauled stack. Therefore, after the overhaul, the brake stack comprises mid-thickness rotors and mid-thickness stators, pressure plate and end plate. After the second service run, at the second overhaul, the disks in the rotor positions are removed and replaced with thick disks. The stators are reassembled into the overhauled brake stack. The overhauled brake stack comprises thick rotors and thin stators, end plate, and pressure plate. After the third service run, at the third overhaul, the thin stators, end plate, and pressure plate are removed and replaced with mid-thickness disks. The brake stack is reassembled with the rotors which have been worn to become mid-thickness disks and mid-thickness stators. Also, at this third overhaul, the pressure plate and the end plate are replaced with mid-thickness disks. Following a fourth service run, at the fourth overhaul, the disks in the stator positions are removed and replaced with thick disks. Similarly, the pressure plate and the end plate are replaced with thick disks. The brake stack configuration after the fourth overhaul but prior to service is the same as the initial brake stack configuration.
By this configuration, a thick disk can go through three separate service runs prior to being replaced, where a mid-thickness disk can go through two service runs prior to being replaced and a thin disk can go through one service run prior to being replaced. If desired, at overhaul, following a service run, the disks can be moved to different positions within the stack or reversed. Alternatively, disks can be removed after a service run in a brake stack or be reinstalled in any other stack, they do not have to go back to the same stack. Therefore, this unique configuration provides for increased longevity of the individual disks of the brake stack, while maintaining the same heat sink envelope. This provides a substantial cost savings for replacement of worn disks since only the thin disks are replaced at each overhaul.
Alternatively, the invention provides additional design flexibility to a brake designer. Where the brake envelope remains constant, the configuration allows an increase in worn heat sink mass and associated increased energy capacity while maintaining the same disk life as in prior known brake stack configurations. If size and weight considerations dominate the brake design constraints, the brake envelope can be reduced. Using this inventive configuration, one can provide a more compact and lower weight brake assembly while maintaining the same disk life. Another alternative benefit of the inventive configuration is an increase in worn heat sink mass and associated energy capacity as well as increased disk life where the brake envelope and minimum disk thickness must remain constant.
In one aspect of the invention, there is provided a disk brake stack having disks with available wear portions of a first thickness, a second thickness and a third thickness. The brake disk stack comprises a first group of disks in overlapping relationship with a second group of disks. Depending upon the build of the stack, the disks of each group have a different thickness compared to the disks of another group. Initially, in one possible embodiment, the first group of disks comprises thick disks, whereas the second group of disks comprises thin disks. After the first service run, at the first overhaul, the second group of disks are substantially fully worn and are removed. The brake stack is reassembled with mid-thickness disks being placed into the positions of the second group of disks. The first group of disks had been worn to become mid-thickness disks. The brake stack is reassembled with the first group of disks and the second group of disks. The brake stack after this first overhaul comprises a first group of mid-thickness disks and a second group of mid-thickness disks. After the second service run, at the second overhaul, the second group of disks are removed and replaced with thick disks. The brake stack is reassembled and comprises a first group of disks that have been worn to be thin disks and a second group of thick disks. After a third service run, at the third overhaul, the first group of disks is substantially fully worn and removed and replaced with mid-thickness disks. The brake stack is reassembled with the first group of disks and the second group of disks, both having an available wear portion of mid-thickness disks. The brake stack comprises a first group of mid-thickness disks and a second group of mid-thickness disks. After a fourth service run, at the fourth overhaul, the first group of disks is removed and replaced with thick disks. The brake stack is reassembled and the configuration of the stack is the same as the initial brake configuration.
In accordance with another aspect of the invention, there is provided a method of assembling a brake disk stack having disks with available wear portions of three different predetermined thicknesses. Initially, in one embodiment, the brake stack comprises a first group of thick disks and a second group of thin disks in axially overlapping relationship with the first group of disks.
The disk(s) are subjected to a number of landings (based on the available wear portions of the disks having the least (thinnest) available wear portions) and then the brake stack is overhauled. At the first overhaul, after this first service run, the first group of disks is worn and become mid-thickness disks; the second group of disks are fully worn and removed. The second group of disks are replaced with mid-thickness disks. The brake stack now comprises a first group of mid-thickness disks, overlapping second group of mid-thickness disks. After a second service run, at the second overhaul, the first group of disks is worn and the disks of this group become thin disks as well as the second group of disks that are worn to become thin disks. The second group of disks are removed and saved since they can be used when thin disks are necessary. Thick disks are placed in the position of the second group of disks.
At the third overhaul, after a third service run, the first group of disks are replaced with mid-thickness disks. The brake stack is reassembled and the first and second group of disks both have available wear portions of the mid-thickness disks. After a fourth service run, at the fourth overhaul, the first group of disks are removed and replaced with thick disks. The brake stack now has the same configuration as the initial configuration.
For a complete understanding of the objects, techniques and structure of the invention, reference should be made to the following detailed description and accompanying drawings.